The present invention relates to a method of bonding a material to a substrate, and more particularly to adhering a flexible material to a rigid substrate using a vacuum forming process.
Vacuum wrapping is a well known art that has become important in many industries. For example, vacuum wrapping is used to produce automotive parts such as interior door panels and instrument panels. Conventional vacuum wrapping processes include three basic steps: (a) heating a sheet of fabric to make the sheet more pliable; (b) transferring the sheet to a vacuum mold; and (c) vacuum molding the sheet over an adhesive-coated substrate so that the fabric adheres to the substrate to form a completed article.
In the heating step, the sheet is placed in a heating module, suspended above and/or below conventional heating elements by a frame that captures the edges of the sheet. The sheet is heated to a temperature and for a time sufficient to render it pliable.
Next, the sheet is transferred in the frame from the heating module to a vacuum mold. With reference to FIG. 1, the sheet 10 is disposed in the mold between platen mold 58 and contoured mold 56, held in frame 11 by clamps 16 and 18 over the substrate 20. The mold includes vacuum ports 44, through which a vacuum source 40 may draw a vacuum. As illustrated in FIG. 2, the mold 50 is closed. The sheet 10 is adhered to the upper contours of the substrate 20 by the adhesive 2 as the sheet is lowered. FIG. 3 depicts the sheet 10 after it has conformed to a majority of the substrate surfaces, excluding recesses 36 and 38. With the mold closed, the clamps 16 and 18 open to release the sheet 10; however, the sheet does not move because its edges remain pinched between platen 58 and contoured mold 56.
In FIG. 3, a vacuum is drawn through vacuum holes 44 and substrate holes 22 to draw the sheet 10 into conformity with the substrate 20. With reference to FIG. 4, the vacuum has drawn the sheet 10 into conformity with the substrate 20, particularly recess 36.
Although the prior art vacuum wrapping process effectively adheres a sheet to a substrate, it suffers a number of shortcomings. First, during the application of the vacuum, the sheet is stretched as it conforms around the edge of the substrate. In some cases, the sheet is torn. Both stretching and tearing and ripping defects are unacceptable. Moreover, it is typically impossible to vacuum wrap thinner, more fragile sheets with the prior art process because such sheets are very prone to tearing.
Second, the high vacuum pulls the sheet into the recesses so rapidly that in some instances the sheet stretches, crinkles, and/or rolls over on itself to form wrinkles in the sheet. FIG. 4 depicts such wrinkles 13. This defect also is unacceptable.
Third, excessively heated sheets become prone to tearing when subjected to a vacuum. Unfortunately, the only way an operator can determine if a sheet has been excessively heated, and thus subject to tearing during vacuum application, is by trial and error. For example, a first sheet torn too easily indicates to the operator that the heat applied to the next sheet must be reduced before vacuum wrapping, or the amount of vacuum used to draw the next sheet must be reduced. If neither of these actions is taken, the next sheet will similarly tear. This trial and error process wastes material and valuable manufacturing time.
Fourth, the prior art vacuum wrapping process requires large margins along the edges of the sheet so that the sheet may be gripped between the platen and the contoured mold before the application of high vacuum. These large margins increase scrap generation.
The aforementioned problems are overcome by the present invention wherein the draw of a sheet by a vacuum is controlled and further wherein a control system senses and controls parameters of temperature and vacuum applied to a sheet to prevent deformation.
In a first aspect of the invention, the free edges of the sheet are controllably released as the sheet is vacuum wrapped into contoured recesses under a vacuum to prevent stretching, and tearing and wrinkling. More specifically, the vacuum mold includes a clamping device operated by a spring or other hydraulic assist mechanism that contacts the margin of the sheet and frictionally retains the sheet against a surface of the mold. The preferred clamping device is a spring-loaded rotatable bar. The spring pushes the bar down against a portion of the sheet, clamping the sheet against the mold. Thus, when the mold is closed, in addition to being held by conventional pinching of the sheet between mold halves, the sheet is also held by friction between the clamping device and the mold.
Accordingly, when the mold halves are opened, and a vacuum is applied to draw the sheet into the contoured recesses of the substrate, the clamping device keeps the sheet in close contact against the mold to prevent reduction of the vacuum and additionally, prevents the sheet from being rapidly pulled into the recesses. Thus, the sheet is drawn into recesses in a slow, controlled manner. Advantageously, this controlled draw reduces stretching and wrinkling of sheets of conventional thickness. Moreover, thinner, more fragile materials may be vacuum molded without tearing.
In an alternative embodiment of this aspect of the invention, the controlled release clamping devices are hydraulically operated and include independent position sensors that act in concert with pressure sensors on the mold. The sensors monitor the draw of the sheet by the vacuum and send this information to a processing unit. The processing unit then alters the clamping force exerted on the sheet by the clamping device or the vacuum to more accurately control the draw of the sheet. Advantageously, this approach prevents unnecessary stretching of the sheet and reduces the time necessary to draw the sheet onto the substrate.
In a second aspect of the present invention, a multi-staged or variable vacuum is used to selectively draw the sheet toward the substrate. The vacuum may be selectively increased or decreased to create a non-constant vacuum profile to further reduce stretching.
In a third aspect of the invention, the temperature of the sheet and the vacuum drawn on the sheet are monitored and controlled to prevent unnecessary tearing of holes in the sheet. It has been discovered that the temperature of a thermoplastic sheet is inversely related to the vacuum that may be drawn on that sheet without unintentionally creating a hole. Thus, sheets heated to a relatively high temperature should be drawn into contours of a substrate with a lower vacuum than sheets that are heated to a relatively low temperature.
In accordance with the above discovery, the present inventive process selectively monitors with sensors parameters such as the temperature of a formable sheet and the pressure of the vacuum used to draw the sheet onto a substrate. The temperature and pressure data are processed by a control unit, and the control unit compares the temperature data and pressure data. Based on the comparison, the control unit alters parameters of the process, such as the heating cycle, the vacuum level, and/or the vacuum timing. For example, if the temperature data indicates that the sheet has been heated to a relatively high temperature, the control unit sends output to the vacuum source to reduce the amount of vacuum drawn on the sheet to prevent tearing. Similarly, if the temperature data indicates that the sheet has been heated to a relatively low temperature, the control unit sends output to the vacuum source to increase the amount of vacuum drawn on the sheet because the sheet requires that additional vacuum to form properly. Additionally, in response to the comparison of temperature and pressure data, the control unit may send output to the heating module to increase or decrease the heat applied to the sheet while keeping the vacuum unchanged.
By using the process of the present invention to monitor and manipulate pressure, temperature, and/or other parameters, less waste is produced.
These and other objects, advantages, and features of the invention will be more readily understood and appreciated by reference to the detailed description of the preferred embodiments and the drawings.